Screen printing is one of the conventional processes for printing images directly onto textiles. Screen printing inks consist of pigments dispersed in an aqueous print paste which contains binder and crosslinkable fixing agent. These mixtures crosslink at a higher temperature after the printing operation, thereby fixing the print on the textile. The several disadvantages of commercial crosslinkable fixing agents include undesirable byproducts, such as formaldehyde, short pot life, and difficult dispersion.
Uhl et. al., U.S. Pat. No. 4,849,262, discloses a printing paste and dyeing liquor containing fine particle dispersions of polyisocyanates in a deactivated form. The deactivation of the particle surfaces is achieved by the dispersion of polyisocyanates in the presence of media which is reactive with isocyanate. Only the isocyanate groups which are present on the surface of the particles react with the deactivating agent. The rest of the polyisocyanate molecules in the interior of the particle remain unreacted. The deactivation compounds form a sort of polymer shell on the surface of the polyisocyanate particles which is removed with heat.
Traubel et al., U.S. Pat. No. 5,556,935, discloses a textile printing paste containing a hydrophilically modified polyisocyanate crosslinking agent. A hydrophilic polyisocyanate prepolymer is used in association with polyepoxide compounds and modified polycarbodiimides. Reiff et al., U.S. Pat. No. 5,607,482, discloses a textile printing paste containing a chemically blocked polyisocyanate crosslinking agent. A hydrophilic polyisocyanate is blocked to prevent reaction. In both of the above cases, aqueous or oil-in-water emulsion print pastes are required due to the hydrophilic nature of the paste components.
Modern lithography is based on modifying the surface properties of coated metal plates. The most common are zinc or aluminum printing plates coated with a light-sensitive oleophilic and hydrophobic material. When the plate is exposed to light through a photographic color separation negative, the exposed areas become "cured" so that the film can be washed off in the unexposed areas. Thus the design becomes reproduced on the plate in a pattern of oleophilic image areas and hydrophilic non-image areas. The image area accepts an oil-based ink and the non-image area does not. In general, the non-image area is constituted by a hydrophilic area accepting water. Accordingly, ordinary lithographic printing is conducted by supplying both a colored ink and an aqueous fount, or fountain ink, to the surface of a printing plate whereby the oil-based ink and the fountain ink are selectively accepted by the image area and the non-image area of the plate, respectively. The process is termed offset lithography because the colored inked image is first offset onto a rubber roller, followed by transfer to paper. The lithographic process is a balance between the properties of the ink, fount, and printing plate.
Common vehicles for lithographic inks include drying oils, synthetic drying oils, rosins, such as copal, dammar, shellac, hardened rosin, and rosin esters, phenolic resins, such as rosin-modified phenolic resins and 100% phenolic resins, maleic acid resins, alkyd resins, petroleum resins, vinyl resins, acrylic resins, polyamide resins, epoxy resins, aminoalkyd resins, polyurethane resins, aminoplasts, cellulose derivatives such as nitrocellulose and ethylcellulose, glue, casein, dextrin, and the like. Other additives generally used in lithographic printing inks include waxes, greases, plasticizers, stabilizers, drying agents, thickeners, dispersants, and fillers.
The ink composition may be prepared by uniformly mixing or kneading the vehicle for the ink, colorant, and additives by an ordinary method such as roll mill method, the ball mill method, the attritor method or the sand mill method.
Fountain inks may contain not only water, but also water modified by such substances as desensitization accelerators, buffers, preservatives, and wetting agents. Examples of such substances are gum arabic, carboxymethylcellulose, sodium alginate, polyvinyl pyrrolidine, polyvinyl imidazole, polyvinyl methyl ether-maleic anhydride copolymers, carboxymethyl starch, ammonium alginate, methyl cellulose sulfates (e.g. sodium sulfate and ammonium sulfate), phosphoric acid, nitric acid, nitrous acid, tannic acid and salts thereof, polyol compounds having two or more hydroxyl groups (polyethylene glycols, ethylene glycol, propylene glycol, glycerol, diethylene glycol, hexylene glycol), organic weak acids (citric acid, succinic acid, tartaric acid, adipic acid, ascorbic acid, propionic acid), polyacrylic acid, ammonium bichromate, alginic ester of propylene glycol, aminopolycarboxylate (e.g. ethylenediaminetetraacetic acid sodium salt), inorganic colloids (e.g. colloidal silica), and surface active agents. These compounds are used each alone or in mixtures. In addition to the above compounds there can be used water-miscible organic solvents such as methanol, dimethylformamide, and dioxane, a small amount of colorants such as phthalocyanine dyes, malachite green, and ultramarines.
Krishnan et al., U.S. Pat. Nos. 5,725,646 and 5,778,789 disclose water-based lithographic printing inks. The main reason for using this type of system is to reduce the volatile organic compounds (VOCs) found in conventional lithographic ink. A water-based lithographic printing ink requires a printing plate with hydrophilic image area and hydrophobic non-image area. If a volatile hydrocarbon fountain solution is required, there will not be a significant reduction of VOCs in the process.
The invention of waterless lithographic printing plates eliminates the use of fountain solutions. The non-image area is coated with a polymer, such as silicon, which is ink repellant. Lint and debris tend to damage the surface of such a plate which limits the life of the plate. The difference in surface energy between the image and non-image areas of conventional offset lithographic printing plates is typically 40 dynes/cm, while that for waterless printing plates is around 20 dynes/cm. This narrower surface energy difference increases scumming, where the non-image area accepts and transfers ink to the blanket and subsequently to the print.
There are many advantages of transfer printing versus direct printing. In transfer printing, the final image may appear on substrates other than those which are easily processed by a printer. Printed images may be transferred onto textiles, such as clothing, whereas direct printing onto the clothing may be problematic. The image may be printed onto a substrate, which acts as an intermediate medium, and stored until use at a later time. The storage time may be indefinite prior to transfer to the final substrate. This is especially advantageous in the garment industry, where fashions change rapidly. Through the use of transfers, printed fabrics are not wasted when styles change. Another advantage of transfer printing is that the printed image may be transferred onto any suitable substrate regardless of shape, size, or composition.
Transfer processes using sublimation, or disperse, dyes are known in the art. See, Hale, U.S. Pat. No. 5,246,518, for example. Sublimation dye solids change to a gas at about 400.sup.- F, and have a high affinity for polyester at the activation temperature. While sublimation dyes yield excellent results when a polyester substrate is used, these dyes have a limited affinity for other materials, such as natural fabrics like cotton and wool.
Accordingly, images produced by heat activated inks comprising sublimation dyes which are transferred onto textile materials having a high percentage of natural fabric as a component, such as cotton, wool or silk, do not yield the high quality image experienced when images formed by such inks are printed onto a polyester substrate. Image transfer, using sublimation dyes and applied heat and pressure, onto substrates of natural fabric, such as cotton, or cotton and polyester blends, yields poor results.
Plate printing processes, and particlularly offset lithography, are the most widely used forms of printing. A need exists for image transfer processes where the image is printed by a plate printing process, and is subsequently permanently transferred to substrates which do not have a polymer or polyester component, such as natural textile fabrics. A long shelf life of the ink prior to final transfer of the image is also a requirement.